The present invention concerns improved capacitors and particularly capacitors employing a pseudo-capacitor-type cathode and a wet slug-type capacitor anode to achieve improved performance including increased energy storage density.
Tantalum wet slug capacitors have long been known in the capacitor arts. An example of the structure of a wet slug tantalum capacitor is described in U.S. Pat. No. 4,780,797. Fundamentally, as described there, the wet slug capacitor includes a tantalum or tantalum-plated container that is the cathode or negative terminal of the electrolytic capacitor. An electrolyte and a porous sintered tantalum anode are disposed within the container. Tantalum forms a native oxide on exposed surfaces that may be increased in thickness by anodic oxidation. In the conventional wet slug capacitor, both the anode and cathode have insulating tantalum oxide coatings that are spaced apart from each other but are both in contact with the electrolyte, typically a sulfuric acid solution. Since sulfuric acid is electrically conductive, a conductor-insulator-conductor structure including metal, oxide coating, and electrolyte is present at both the anode and the cathode. Each of these conductor-insulator-conductor structures is itself a capacitor, i.e., an anode capacitor and a cathode capacitor. The capacitances of these electrode capacitors are to some degree determined by the thickness of the oxide layers formed on the anode and the cathode. Increasing the thickness of the anode oxide layer but not the cathode oxide layer, for example, by anodic oxidation, increases the breakdown voltage that a wet slug capacitor can withstand but reduces the overall capacitance of the capacitor. Typical breakdown voltages for a single capacitor can range from ten to one hundred twenty-five volts.
In the wet slug capacitor, the anode capacitance is effectively electrically connected in series with the cathode capacitance. As is well known, the net capacitance of two capacitors connected in series is smaller than the smaller of the capacitances of the two capacitors. Because the oxide layer at the anode of a wet slug capacitor is usually much thicker than the thickness of the oxide layer at the cathode, the anode capacitance of a wet slug capacitor is smaller than the cathode capacitance. For example, in a typical structure, the anode capacitance may be 3,100 microfarads and the cathode capacitance may be 8,700 microfarads. The resulting net capacitance of that capacitor is about 2,300 microfarads.
Although wet slug capacitors having useful capacitances and breakdown voltages can be readily produced, there is always a desire to increase the capacitance per unit volume of those capacitors, i.e., the energy storage density, without a reduction in the breakdown voltage. One proposed method of increasing the energy storage density of a wet slug capacitor is described in the cited patent. In that patent, a number of metallic films are deposited on the inside of the container of the capacitor. In particular, it is suggested that a film selected from the platinum group of metals, i.e., ruthenium, rhodium, palladium, and platinum, be alloyed with the tantalum of the container in segregated islands where the native oxide has been removed from the tantalum. Various techniques can be employed to deposit the platinum group metal, such as sputtering and electrolytic or electroless plating, followed by a heat treatment at a relatively high temperature, for example, from 925xc2x0 C. to 1,500xc2x0 C. Preferably, a platinum group metal layer is subsequently deposited on the islands to form a spongy layer. The platinum group metals apparently improve the energy storage density of capacitors having the structure described in the patent.
In U.S. Pat. No. 4,942,500, a platinum group metal is applied to a capacitor cathode by cladding, i.e., by rolling a very thin layer of the platinum group metal with the tantalum metal. Explosive bonding is also mentioned. In U.S. Pat. No. 5,043,847, electrolytic co-deposition of a base metal and platinum group metal on the inside surface of a wet slug capacitor container is described. Addition of the platinum group metal by these techniques is said to increase the energy storage density.
A different type of electrolytic capacitor, frequently referred to as an electrochemical capacitor, employs so-called pseudocapacitive electrodes. These capacitors generally have metal oxide electrodes including a substrate of titanium or tantalum. Typically, a hydrated chloride of the metal, which may be ruthenium, is dissolved in isopropyl alcohol and applied to a heated titanium or tantalum substrate. The heat drives off the solvent, resulting in the deposition of a metal chloride. That chloride is heated to a high temperature in air to convert the metal chloride to an oxide. For example, the metal chloride film may be heated to about 250xc2x0 C. for approximately one-half hour to remove the solvent completely and to drive off water. Thereafter, in a second elevated temperature step, for example, at approximately 300xc2x0 C., a large surface area film of the oxide of the metal, for example, ruthenium oxide, is formed on the substrate. The oxide film is highly porous, meaning that it has a very large surface area.
A conventional electrochemical capacitor includes such electrodes as the anode and as the cathode, typically with a sulfuric acid solution electrolyte. The electrical charge storage mechanism is not fully understood. Electrical charges may be stored on the very large surface areas of the two electrodes, providing the capacitance characteristic. Electrical charges may be stored by a reversible change in the oxidation state of a material in an electrode. No matter what the charge storage mechanism is, it is substantially different from the charge storage mechanism of a wet slug capacitor electrode.
Although electrochemical capacitors can provide much higher energy storage densities than wet slug capacitors, the breakdown voltage of individual cell electrochemical capacitors is very low, typically only about one volt, i.e., essentially the dielectric breakdown voltage of the electrolyte. Even if electrochemical capacitors are connected in series, it is difficult to produce a practical capacitor with a breakdown voltage comparable to the breakdown voltages of wet slug capacitors. Thus, electrochemical capacitors have not been widely used.
It is an object of the invention to provide an improved capacitor having a practical breakdown voltage and a high energy storage density.
According to one aspect of the invention, a capacitor comprises a cathode including a coating of an oxide of at least one metal selected from the group consisting of cobalt, molybdenum, and tungsten, an anode spaced from the cathode and including a metal selected from the group consisting of tantalum, aluminum, niobium, zirconium, and titanium, and an electrolyte disposed between and in contact with the cathode and the an-ode.
According to a second aspect of the invention, a capacitor comprises a cathode including a coating of a nitride of a metal selected from the group consisting of titanium, vanadium, zirconium, niobium, molybdenum, and tungsten, an anode spaced from the cathode and including a metal selected from the group consisting of tantalum, aluminum, niobium, zirconium, and titanium, and an electrolyte disposed between and in contact with the cathode and the anode.
According to a third aspect of the invention, a capacitor comprises a cathode including a coating of a carbide of a metal selected from the group consisting of vanadium, niobium, molybdenum, and tungsten, an anode spaced from the cathode and including a metal selected from the group consisting of tantalum, aluminum, niobium, zirconium, and titanium, and an electrolyte disposed between and in contact with the cathode and the anode.
According to another aspect of the invention, a capacitor comprises a carbon intercalation cathode, an anode including a metal selected from the group consisting of tantalum, aluminum, niobium, zirconium, and titanium disposed on a second metal body opposite and spaced from the carbon intercalation cathode, and an electrolyte disposed between and in contact with the cathode and the anode.
According to a further aspect of the invention, a capacitor comprises an electrically conducting polymeric cathode selected from the group consisting of polyacetylene, polypyrrole, poly-p-phenylene, polythiophene, poly-3-phenylthiophene, and poly-3-(4-fluorophenyl)-thiophene, an anode including a metal selected from the group consisting of tantalum, aluminum, niobium, zirconium, and titanium disposed on a second metal body opposite and spaced from the electrically conducting polymeric cathode, and an electrolyte disposed between and in contact with the cathode and the anode.
In the invention, one electrode of a capacitor is a wet slug capacitor-type electrode, for example, the anode. The other electrode is an electrochemical-type capacitor electrode, for example, an oxide of cobalt or tungsten, an oxide or nitride of molybdenum, a nitride of titanium, vanadium, zirconium, niobium, molybdenum, or tungsten, a carbide of vanadium, niobium, molybdenum, or tungsten, a carbon intercalation cathode, or an electrically conducting polymer. For the same capacitance value, the cathode of a capacitor according to the invention is reduced in size compared to a conventional wet slug capacitor. If the volume of the conventional wet slug capacitor for a particular capacitance is maintained, then the anode of a capacitor according to the invention can be increased in size relative to the conventional wet slug capacitor anode, increasing the capacitance and the energy storage density as compared to a conventional wet slug capacitor. In addition, if the capacitance of a conventional wet slug capacitor is maintained, then the volume of a corresponding capacitor according to the invention can be made smaller than the conventional wet slug capacitor, increasing energy storage density. A high breakdown voltage, characteristic of the conventional wet slug capacitor, is obtained in the invention, because of the presence of the conventional wet slug capacitor anode, while realizing increased energy storage density because of the presence of the pseudocapacitor cathode.